Broadband powerline communication (PLC) is a technology that modulates a radio signal with data and transmits the signal on existing electricity powerlines in a band of frequencies that are not used for supplying electricity. In particular, current PLC systems transmit signals at relatively low frequencies (i.e. <30 MHz). High maximum throughput is achieved by employing modulations with a large number of bits per second per carrier per hertz (i.e. bpc/Hz) in the PLC modulation schemes. These systems typically define maximum and useable bps/Hz for each carrier. The transmitter and receiver are often capable of negotiating the used bpc/Hz for each carrier according to a received Signal to Noise Ratio (SNR) so as to optimize the channel capacity between any two nodes (within the limits of the defined modulation parameters). The available channel capacity between any two nodes of the powerline differs with the frequency of a transmitted signal (because of different attenuations, effects of multi-path delays and clock accuracy and noise). The available dynamic range of the implementation and the defined parameters of the modulation in the communication scheme also limit the achieved throughput.
The Federal Communications Commission (FCC) establishes limitations on the conducted and radiated emissions from electronic devices. Conducted emissions are currents that are passed out through a power cord and placed on a common power net, where they may radiate more efficiently because of the larger expanse of this antenna, thereby interfering with other devices. The frequency range for conducting emissions is 150 KHz to 30 MHz. Radiated emissions are electric and magnetic fields radiated by a device, wherein these emissions may be received by other electronic devices causing interference therewith. The frequency range for radiated emissions extends from 30 MHz to 40 GHz. There are other regulations in different regions, some specifically for powerline and some for more general applications and some set by different regulatory bodies.
In practice, power lines are neither shielded nor well-balanced. Thus, some of the RF energy they carry is radiated there from. This RF “leakage” can interfere with licensed radio services. Thus, PLC operators are often required to attenuate or “notch” PLC signals in frequency bands where licensed services are in nearby use. Furthermore, some of the regulations and standards require the injected power spectral density (PSD) of the radio signal in a PLC system to be below approximately −80 dbm/Hz in these notches and above 30 MHz, even through a significantly higher (e.g. up to −50 dBm/Hz) PSD can be injected outside the notches and/or below 30 MHz.
For simplicity, the term “notch” refers to a frequency band where the energy level of a PLC signal has been deliberately reduced to prevent interference with other users of the spectrum. The term “sub-band” refers to a frequency band where a PLC signal characteristic differs (e.g. in power level or directionality) from those in the rest of the PLC signal's bandwidth. The term “coverage” refers to the maximum distance between two nodes at which data transmitted there between is still detectable. In PLC it also refers to the percentage of node pairs that can communicate, to a given minimum performance. Similarly, the term “throughput” refers to the rate at which nodes send or receive data on a network. Coverage in a network is generally dominated by a maximum injected power, as node pairs that have difficulty communicating, do so primarily because of channel attenuation and receiver noise. However there are many node pairs where the throughput between them is not limited by the channel, but by the implementation of the communication system.
In future PLC systems (such as those being defined by IEEE P1901 and ITU-T Ghn), there is a desire to increase throughput, whilst maintaining or improving coverage, keeping a reasonable implementation cost and meeting regulatory requirements. One approach to increase throughput, is to increase the bandwidth of the single band starting below 30 MHz to make it go above 30 MHz. However, the severe stepped PSD (difference of ˜30 dB) imposed by the above regulations, makes it difficult to use a communication band extending above and below 30 MHz, at a reasonable implementation cost, because the dynamic range of both the transmitter and receiver must be capable of handling the step in signal power.
The PSD step between the largest and smallest sub-bands within a band and the defined maximum bpc/Hz for each of the carriers largely defines the dynamic range requirements of the transmitter and receiver. The PSD step further drives the level of quantization, noise and linearity required to maintain signal integrity. Linearity, noise and quantization requirements significantly affect the implementation costs of the analog and digital sections of a modem, and lead to practical limits which may make it too expensive or impossible to implement the same maximum bps/Hz in the sub-bands with a lower PSD.
In principle, any PLC system may support multiple carriers operating in separate sub-bands operating with different injected power levels. It may also use only a subset of the carriers to communicate with nodes using only a part of the communication band (e.g. legacy systems extending over 1.8 Mhz to 30 MHz). It may also be part of a system where another independent band is used in parallel in the same medium or across another medium.